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Refractive index sensitivity of the UltraPath 2 sample cell
- salinity matched reference solutions to avoid baseline offsets
The UltraPath 2 sample cell is sensitive to variations in refractive index between reference and
sample solution. When measuring in saline solutions, such as e.g. colored dissolved organic
matter (CDOM ) in seawater, it is recommended to use a reference/standard solution of
matched salinity. These reference solutions should be filtered with the same filter material as
the sample (e.g., 0.2-micron filters).
Saline solutions have a slightly higher refractive index that pure water, resulting in a negative
absorbance baseline shift compared to de-ionized water. For more information see:
Richard L. Miller, Mathias Belz, Carlos Del Castillo, Rick Trzaska,
“Deter
mining CDOM Absorption Spectra in Diverse Coastal Environments
Using a Multiple Pathlength, Liquid Core Waveguide System”,
Continental
Shelf Research
(July 2002), 22:9, p 1301-1310.
Measurement reliability
Baseline absorbance measurements should be within ± 1-2 mAU between baseline
samples. If larger baseline variations are observed, the following problems may be
present:
1.
An air bubble is trapped inside the cell. Allow for more sample to be injected and
scan again.
2.
The cell is contaminated. Follow the cleaning protocol and take a new reference.
3.
The salinity of the sample is not the same as the salinity of the reference solution. If
working in saline environments, prepare saline-matched reference solutions to
accommodate for baseline shifts.
See instrument maintenance and troubleshooting section on how to overcome these
problems.
Flow rate and maximum pressure
The applied pressure and fluid flow rate through the LWCC /UltraPath 2 obeys the Hagen-
Poiseuille relationship. Flow is proportional to pressure and to the fourth power of the
diameter of the sample cell, as well as reciprocal to the length of the cell and fluid viscosity.
Typical flow rates of the UltraPath 2 sample cell are 12 mL/ min (equal to 60 RPM with the
Peri-Star Pro pump using size #14 tubing). Drawing liquid into the cell via the injector fills the
sample cell. The UltraPath 2 sample cell is designed for low pressure operations and should not
be used at pressures greater than 75 PSI.
